Dual fuel injector with purge and premix

ABSTRACT

A fuel injector for a gas turbine conbustor, the injector operating on gas and liquid fuels selectively. The injector has a central liquid fuel duct and jets, an annular gas duct and gas jets, and an outer annular combustion air duct. Operation on the liquid fuel tends to cause combustion products to be ingested into the gas orifices so impeding efficient gas combustion. The invention provides apertures (17) between the outer air duct and the intermediate gas duct whereby during operation on liquid fuel air bleeds into the gas duct and purges the gas orifices. As an added advantage, in a transition from liquid fuel to gas, the gas pressure is increased to a point where the air bleed is reversed and gas bleeds into the air duct. The pre-mixed gas/air is then emitted from swirlers surrounding the ring of gas jets. More efficient gas combustion results.

BACKGROUND OF THE INVENTION

This application is a 371 of PCT/GB94/02219 filed Oct. 12, 1994.

This invention relates to gas turbine engines which operate on gas andat least one alternative fuel. A typical application is to a dual fuelgas turbine operating on gas as the primary or main fuel and liquid asthe secondary or stand-by fuel.

During the operation of a dual fuel gas turbine on the stand-by fuel,liquid for example, the gas fuel injector passages are open to the hotcombustion products of the primary zone. Hence re-circulation of the hotgases within the gas injector passages is inevitable. This problem isexaggerated if the gas passages in question are designed for lowcalorific value (LBTU) gas fuel and are therefore larger than thosedesigned for natural gas operation.

The ingestion of combustion products may cause damage or blockage of thegas passages. Previous work addressed this problem by purging the gaspassages when operating on the stand-by fuel by the use of externalpressurised steam or other gases. This method although effective,necessitates the addition of expensive equipment to generate and orsupply the purge medium therefore increasing both capital and operatingcosts.

While the use of compressor air for this purging process has beenproposed previously, difficulties involving fuel spilling back (i.e.upstream of the injector nozzle) and flashback have not previously beenovercome.

SUMMARY OF THE INVENTION

An object of the present invention is to alleviate the abovedifficulties while benefitting the control of smoke emissions.

According to the present invention, a fuel injector for a combustor of aturbine engine operable on either or both of primary and secondary fluidfuels, in which primary fuel orifices are exposed to combustion productsduring operation on the secondary fuel, comprises secondary fuelorifices connected to a secondary fuel supply passage, a ring of primaryfuel orifices connected to an annular primary fuel manifold, and anannular air passage providing combustion air for fuel injected by theprimary and secondary fuel orifices, the air passage being formedbetween a wall of the manifold and a shroud member and having an inletfor combustion air between an inlet end of the shroud member and thewall of the manifold, the injector further comprising a multiplicity ofholes in the manifold wall between the air passage and the manifold, theholes and the air passage dimensions being such that the primaryorifices are purged by the emission of air during operation on secondaryfuel and, during operation on primary fuel, at low fuel pressure airbleeds through the holes and at high fuel pressure primary fuel bleedsthrough the holes to provide in both cases a pre-mix of primary fuel andair.

The primary fuel manifold may comprise an intermediate wall dividing thedownstream end of the manifold into concentric annular regions, theprimary fuel orifices opening into the inner one of these regions andthe outer one of these regions comprising turbulence inducing means.

The air passage may be open to a combustion region downstream of theinjector by way of turbulence inducing means.

The fuel injector may comprise a cylindrical fuel orifice head with aplanar downstream face having an array of secondary fuel orificessurrounded by a ring of primary fuel orifices the secondary fuelorifices having access to an axial secondary fuel duct and the primaryfuel orifices having access to an annular primary fuel passage, theshroud member comprising a substantially cylindrical portion and anannular portion having an aperture providing access to a combustionregion downstream of the injector, the annular portion being spaced fromthe downstream face to provide a path for purging air in operation onsecondary fuel and for pre-mix combustion air in operation on primaryfuel.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiments of fuel injector in accordance with the invention willnow be described, by way of example, with reference to the accompanyingdrawings in which:

FIG. 1 is an axial section of an LBTU dual fuel injector; and

FIG. 2 is an axial section of a natural gas dual fuel injector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The injector is mounted in a combustion chamber (not shown) and may beone of a number of similar injectors mounted in an annular arrangementfacing downstream. Upstream of the injectors is a source of combustionair, i.e. an air compressor of the gas turbine engine incorporating thecombustor. Compressed air is driven past (and as will be seen, through)the injectors basically to permit combustion in the `primary zone` 2downstream of the injectors.

Referring to FIG. 1, the primary fuel, gas is fed to an annular gaspassage or manifold 1 which feeds a ring of gas orifices 3. The gaspassage 1 also feeds an annular array of guide vanes which act as`swirlers` 5 to give a rotational deflection and a degree of turbulenceto the emergent fuel or fuel-air mixture about the injector axis. Theswirled gas component is separated from that through the orifices 3 byan intermediate wall 7.

Surrounding the gas passage is a shroud 9 which is open to the upstreamend to gather in compressor air and pass it out at the downstream end tomix with fuel from the adjacent orifices 3 and the swirlers 5. Thedownstream end of the shrouded air passage 13 is fitted with furtheraxial swirlers 15 to improve the mixing of the gas fuel and compressorair.

The outer wall 11 of the gas passage 1 has a number of radial holes 17around its circumference at an axial position just upstream of theupstream end of the intermediate wall 7 of the gas passage 1. Compressorair can therefore enter the gas passage by way of the purging holes 17in the absence of gas fuel and emerge from the gas orifices 3.

Within the inner wall 19 of the gas passage 1 is a secondary fuel nozzle21 which typically operates with liquid fuel. This is supplied by anaxial duct 23 and is injected into the combustion chamber from orifices25. In operation on liquid fuel, combustion products close to theinjector would tend to circulate and enter the gas orifices 3. Solidswould be deposited and the efficiency of the combustor reduced:Overcoming this disadvantage is one of the aims of the invention.

When operating on gas fuel the gas fuel pressure is increased from zeroand the flow or bleeding of air from the air passage 13 through theapertures 17 is consequently reduced as the opposing fuel pressureincreases. Above this `pressure balance` condition the increased gaspressure reverses the flow direction to cause gas bleeding through theapertures 17 to spill into the air passage 13. Such spillage is however,prevented from spreading into the air stream upstream of the aperturesby the shroud 9 which confines the spillage to the fast flowing airstream close to the injector wall 11. The spilled fuel and air isfurther mixed by the swirlers 15 on emerging into the combustion zone.Spillage of the gas fuel is therefore prevented from moving upstream andcausing ignition flashback which might otherwise occur, in addition toproviding a pre-mix of fuel and air.

In addition to the above purge and spillage/flashback considerations theprovision of purging air during operation on the secondary (liquid) fuelresets the primary zone (2) stoichiometry advantageously. A leanermixture is produced which is beneficial to smoke emissions control.Furthermore, on fuel change to LBTU gas fuel, which, as explained above,eliminates the purge air flow, the primary zone stoichiometry becomesrelatively richer, which is beneficial to low power carbon monoxideemission control.

An injector of the form described can also be used to reduce the NOxemissions of the combustion process by increasing the aperture (17) sizeto allow a larger portion of the gas fuel to exit through the apertureto the air passage 13 and swirlers 15. This portion will be partiallymixed with swirler air and this will have the same effect as reducingthe calorific value of the gas fuel and result in a reduction in NOxemissions.

The advantage of using this method for NOx control over premixed systemswith pilot fuelling for starting and flame stabilization is thesimplicity of both fuel and control system as only one gas fuel manifoldand one gas flow control are required.

At starting and low load the gas flow will occur from the original gasholes 3 and as, in these conditions, the pressure is low in the gaspassages it will only be capable of overcoming the combustion chamber(primary zone 2) pressure which is lower than the compressor deliverypressure in the air passages 13. At a given-operating load condition therequired gas flow will necessitate an increase in gas pressure above theair pressure in the air passages 13 therefore allowing gas to exit fromthe purge aperture 17 mixed with the air. The operating point at whichthis is achieved can be set by the design parameters of the apertures,the air passages and the combustor pressure drop. The operating rangeover which this process occurs can be chosen to cover startingconditions only or any intermediate range up to the full speed no load(`FSNL`) point.

FIG. 2 shows a fuel injector suitable for natural gas fuel as opposed tothe LBTU gas of FIG. 1. The form of the injector is different but thebasic elements of the FIG. 1 design are present. Thus a shroud 9surrounds an annular gas passage 1 the outer wall 11 of which has a ringof purge apertures 17. The gas passage 1 terminates in a flat head witha ring of gas orifices 3. The shroud 9 is combined with a swirler head14 which has a central aperture 16 providing access for the gas jets tothe combustion chamber. Radial swirlers 18 are mounted on the swirlerhead to provide lateral dispersion and mixing of fuel and air.

The liquid fuel is supplied along an axial bore 23 to liquid fuelorifices 25 as before. Again it is important that while operating onliquid fuel the gas orifices 3 do not become fouled, i.e. coated withcombustion products. This is achieved by arranging that the swirler head14 is spaced from the fuel orifice head by a small gap referred to asthe anti-carbon gap (20).

During operation on the secondary liquid fuel, air is entrained in theair passage 13, passes through the apertures 17 into the gas passage 1and exits from the gas orifices 3 thus preventing the ingress ofcombustion products. Air also passes through the air passage 13 outsidethe gas passage and through the anti-carbon gap 20.

It may be seen that the principles of operation are the same as for theFIG. 1 embodiment. Spillage and flashback during operation on theprimary fuel are inhibited by the constrained air flow through the airpassage 13. Fouling of the gas orifices during secondary fuel operationis prevented by purging air passing through the purge apertures 17 andthrough the gas orifices 3.

I claim:
 1. A fuel injector for a turbine-engine combustor, the injectorcomprising, working radially outwards from an axis:a) a secondary-fuelsupply passage connected to a plurality of secondary-fuel outletorifices, said secondary-fuel outlet orifices opening into a combustionregion downstream of the injector; b) an annular primary-fuel manifoldconnected to a ring of primary-fuel outlet orifices, said primary-fueloutlet orifices opening into said combustion region; c) an annular airpassage for providing combustion air for fuel injected by said primary-and secondary-fuel orifices, said air passage being formed between awall of said primary-fuel manifold and an outer shroud member, said airpassage having an inlet for combustion air between an inlet end of saidshroud member and said primary-fuel manifold wall and an outlet openinginto said combustion region; d) purge and premix means, including:i) amultiplicity of holes in said manifold wall between said air passage andsaid manifold, said holes allowing air to pass into said manifold forthe purging of said primary-fuel outlet orifices during operation onsecondary fuel and, during operation on primary fuel, allowing bleedingof a working fluid of either of said air passage and said manifold intothe other of said air passage and said manifold in dependence onprimary-fuel pressure, thereby in either case to provide a pre-mix ofprimary fuel and air, and ii) means for determining a point during saidoperation on primary fuel at which bleeding of one of said workingfluids gives way to a bleeding of the other of said working fluids. 2.The fuel injector according to claim 1, wherein said determining meansis constituted by at least said holes and said air passage being ofdimensions calculated in dependence on expected values of pressure ofair inside said air passage and of primary fuel inside said primary-fuelmanifold for a given load condition, thereby to effect a transition ofsaid bleeding of one of said working fluids to said bleeding of theother of said working fluids at an appearance of said load condition. 3.The fuel injector according to claim 1, wherein said primary-fuelmanifold includes an intermediate wall dividing a downstream end of saidmanifold into inner and outer concentric annular regions, saidprimary-fuel outlet orifices opening into said inner annular region andsaid outer annular region further including turbulence-inducing means.4. The fuel injector according to claim 1, wherein said outlet means ofsaid air passage includes turbulence-inducing means.
 5. The fuelinjector according to claim 1 adapted for use with a low calorific valueprimary gas fuel, thereby excluding the use of natural gas, and a liquidsecondary fuel.